Effect of Al content on inclusions in the automobile high strength steel

With the addition of Al, the type, amount and precipitation temperature of inclusions in high strength steel for automobile will change.The characteristics of inclusions including amount, species and precipitation temperature in three typical automotive high strength steels with different Al contents were investigated by oxygen and nitrogen analysis, scanning electron microscopy (SEM) , and thermodynamic calculation.The results show that the mass fraction of Al in steel increases from 0.054%to 1.35%, the mass fraction of TOin steel decreases from 0.00150%to 0.00075%, and the number of inclusions decreases from 38.28 mm^-2to 28.12mm^-2, the number of inclusions in steel tends to decrease.With the increase of Al mass fraction in the steel, the evolution route of main oxide inclusions is Al₂O₃·MnO→MgAl₂O₄, the main aluminum inclusion changes from Al₂O₃to Al N;and the evolution route of sulfide inclusions is MnS to MgS.The thermodynamic calculations indicate that the formation of Al N inclusions is related to the Al content in the steel, and the Al N inclusions in steel with high Al content are easier to be formed.The precipitation temperature of Al N increases with the increase of Al content, and the precipitation temperatures are 1375, 1528 and 1561 K, respectively.